1. Field of the Invention
This invention relates to carbazole diazonium salts and the use of such carbazole diazonium salts in pre-polymer or polymer compositions as sources of photo-initiated strong acids. Diazonium salts are used as photoinitiators in such developments as imaging films where strong acids are needed to effect polymerization of the film or to remove protective groups.
2. Prior Art
The prior art discloses the use of diazonium salts as sources of photogenerated strong acid. However, when these diazonium salts are formulated with epoxy monomers or pre-polymers, the mixtures result in premature polymerization. The mixtures thus result in unstable compositions. Various measures have been used to overcome the problem of premature polymerization.
U.S. Pat. No. 3,721,616 discloses a process whereby polymerization of epoxide monomers and pre-polymers, and of other materials that are polymerizable through the action of cationic catalysts, is controlled in compositions essentially free of volatile solvents, by providing in such compositions, in association with a radiation-sensitive catalyst precursor, a gelation inhibitor in the form of a nitrile compound such as acetonitrile present in small amounts up to several percent by weight. The patent discloses that without the use of a nitrile, epoxide and related compositions containing photosensitive catalyst precursors gel upon standing, even in the absence of an energy source such as light or ultraviolet radiation. This tendency to undergo premature reaction is particularly troublesome in the case of formulations which are effectively free of unreactive liquid diluents or solvents and results in decreased shelf life and utility. However, the use of a nitrile as a gelation inhibitor is not completely satisfactory, because the viscosity of the diazonium salt/epoxide mixture increases over time.
U.S. Pat. No. 3,721,617 discloses a process whereby polymerization of epoxide monomers and pre-polymers, and of other materials polymerizable through the action of cationic catalysts, is controlled by providing, in conjunction with a radiation-sensitive catalyst precursor, a gelation inhibitor in the form of a cyclic amide. The presence of the cyclic amide reduces the tendency of the composition to gel upon standing, even in the absence of light or ultraviolet radiation. However, the solutions are not stable because the viscosity of the diazonium salt/epoxide mixture increases with time. The polymerization reaction is exothermic and, where large masses are involved, can generate sufficient heat to cause combustion of the epoxide resins.
U.S. Pat. No. 3,960,684 provides stable diazonium catalyst solutions in which the catalyst is dissolved in an organic sulfone. The catalyst solutions have extended shelf life while retaining the ability to rapidly cure epoxy resins upon exposure to an energy source. However, the solutions are not stable because the viscosity of the diazonium salt/epoxide mixture increases with time.
The three inhibitors, i.e. nitriles, amides and sulfones act as weak bases to remove part of the acid formed in a thermal Schiemann reaction of the diazonium salt. The thermal Schiemann reaction can occur in the dark. The gelation inhibitors cannot stop this reaction; they only delay or reduce the effect of the acids produced.
Gilbert T. Moran and Hugh Norman Read, J. Chem. Soc., 121, 2709 (1922) disclose that carbazole-3-diazonium salts are stable to the reaction conditions by which they are prepared when compared with the corresponding diazo derivatives of the benzene, diphenyl and naphthalene series. For instance, 3-amino carbazole can be diazotized at 0.degree.-8.degree. C. and N-ethyl-3-amino carbazole can be diazotized at 15.degree.-20.degree. C. In addition, they report that the action of light on N-ethyl-carbazole-3-diazonium chloride was inappreciable.
Contrary to Moran and Read, Applicants have found that the carbazole diazonium salts described are very sensitive to light.
The present invention overcomes the need for the addition of a gelation inhibitor by using carbazole diazonium salts as radiation-sensitive catalysts. These compounds have been found to be thermally stable in the pre-polymer or polymer composition in the absence of light.